1. Field of the Invention
The present invention relates to a pigment dispersion, an ink composition containing the pigment dispersion and a printed material.
2. Description of the Related Art
In the case of using a pigment as a colorant in various coloring compositions, it is important to ensure the dispersibility and the dispersion stability of the pigment as a solid. A curable composition or an ink composition having a uniform hue can be obtained by using a pigment dispersion excellent in pigment dispersibility and dispersion stability. Particularly, a pigment of excellent in light fastness is used generally as a colorant for a ink composition. However, in a case where the pigment involves a problem in view of the pigment dispersibility, this causes various problems, for example, of rendering the tone uneven or lowering the dischargeability in a case in which an ink composition is used for an inkjet recording method.
In recent years, an inkjet recording method has attracted attention as an image recording method of forming images on a recording medium based on image data signals. The inkjet recording method has advantages of less generating noises, and is also capable of recording highly fine images at a low running cost by spotting extremely small liquid droplets.
According to the inkjet recording method, while printing is possible not only to plain paper but also to a none-liquid-absorbing recording medium such as a plastic sheet or a metal plate, it is desired to shorten the time necessary for drying and curing for attaining higher speed and higher image quality upon printing. For this purpose, one of inkjet methods involves a recording method of using an ink that can be cured in a short time by irradiation of actinic energy rays as an inkjet recording ink. According to this method, clear and sharp images can be formed by irradiating actinic energy rays just after printing thereby curing ink droplets. For such a curable inkjet composition, high pigment dispersability and aging stability thereof are demanded for forming highly fine images of excellent coloring property and stably discharging the ink composition.
For providing the ink composition with a clear tone and a high coloring power, it is essential that the pigment is made finely particulate. Particularly, in an ink composition used for inkjet recording, since discharged ink droplets give a significant effect on the clearness and sharpness of images, the amount of the discharged liquid droplets is decreased and it is essential to use particles having finer diameter than the thickness of the cured ink film formed from the ink composition. However, as the pigment particles are made finer for obtaining a higher coloring power, fine particles becomes difficult to be dispersed and aggregates are likely to be formed. Further, this also results in a problem such that viscosity of the ink composition is increased by excessive addition of a dispersing agent. Since generation of pigment aggregates and increase in the viscosity of the ink composition gives an adverse effect on the ink dischargeability (ejectability), it is not preferred for inkjet recording to use an ink composition causing pigment aggregates or increase of viscosity.
A method of manufacturing an ink composition containing a fine pigment dispersion includes a method of previously preparing a concentrated pigment dispersion (also referred to as a mill base) and diluting the obtained mill base with a desired liquid such as a solvent or a polymerizable compound, thereby obtaining an ink composition. However, since the mill base is a concentrated dispersion of pigments, the pigments strongly interact with each other, so that viscosity tends to increase during dispersion or storage. Increase in viscosity of the mill base is not preferred since this causes increase in the burden on a manufacturing machine, as well as results in lowering of the manufacturing stability due to circulation failure or lowering of the re-dispersibility upon ink preparation. From the view point described above, there is requirement to develop a mill base having sufficient fluidity and storage stability, and an ink composition used for inkjet having sufficient fluidity, in which a finely particulate pigment is dispersed stably, and which is also excellent curability.
For the dispersing agent for obtaining a stable ink composition, various proposals have been made as described below. That is, an ink composition using a pigment derivative as a dispersing agent for improving the affinity with a pigment (for example, refer to Japanese patent application laid-open (JP-A) Nos. 2003-119414 and 2004-18656); an ink composition using a polymer having a basic group as a dispersing agent for a specified pigment such as a phthalocyanine-type or a quinacridone-type (for example, refer to JP-A No. 2003-321628); an organic solvent-free ink composition containing a dispersing agent such as a poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer together with a specified monomer for dissolving the dispersing agent (for example, refer to JP-A No. 2004-131589); an ink composition using a graft copolymer having a heterocyclic residue that forms an organic pigment as a dispersing agent (for example, refer to JP-A No. 2007-9117); an ionic compound-containing powder dispersion using a pigment dispersant having a basic end and another pigment dispersant in combination (for example, refer to JP-A No. 2006-232987); and a pigment dispersion composition using a pigment dispersant having an acidic group and a pigment dispersant having basic group in combination (for example, refer to JP-A No. 2000-256570) have been disclosed.
The pigment dispersant, the pigment dispersion or the ink composition disclosed in these documents described above may enable to disperse pigments more finely and the stability of the ink composition may be more improved than previous ones, but there is still a room for the improvement of fluidity of the ink composition in a high concentration range and stability of the pigment ink composition. Further, with respect an ink composition having excellent stability, improvement in stability of the mill base has been required.
The present invention aims to address above-described problems and to achieve the following objects.
An object of the invention is to provide a pigment dispersion which contains a finely dispersed pigment and has excellent fluidity in a high concentration region and excellent dispersion stability under a diluted state even during a long-term storage.
Another object of the invention is to provide an ink composition in which pigments are finely dispersed and dispersion stability of the pigment is excellent even after a long-term storage or a repeated change of temperature, so that the ink composition is favorable for inkjet recording, and to provide a printed material produced by using the same.
The inventors of the invention have devotedly and extensively conducted investigations, and as a result, they found that a pigment dispersion having excellent dispersibility and dispersion stability of the pigment is obtained by using a combination of two types of specific polymers and an ink composition capable of effective suppression of decrease in dispersion stability that is caused after a long-term storage or even after a repeated change of temperature is obtained by using the pigment dispersion. Thus, the invention has been completed.